This invention relates to transponders, and more particularly, to transponders including means for modulating the level of power produced thereby.
A transponder is a device that produces an output signal in response to an incoming signal. One form of transponder is a system for providing false returns from an object to a radar system. A conventional technique for providing false azimuth information to radar systems is inverse gain modulation. In utilizing this technique, a radar pulse is transmitted, the signal is received by an aircraft, reproduced and returned to the radar system. The system produces a returned signal having a peak level versus angular displacement corresponding to the lowest level versus angular displacement of the transmitted radar signal, and a lowest level corresponding to the peak level of the transmitted radar signal. This returned signal provides a flat, meaningless return in azimuth, denying the radar system angular position information. (It is noted that the signal returned by the electronic system is greater in magnitude than the return from the remainder of the aircraft, so that the radar system does not respond to actual returns from the skin of the aircraft.)
Prior systems implement the inverse gain technique by receiving incoming signals and storing them in a computer. The signals are analyzed for characteristics such as frequency and pulse repetition frequency. In response to the analysis, a fabricated signal is generated and transmitted back to the radar system. Among the disadvantages of prior systems is the initial delay of response. In the brief time in which the computer is storing and analyzing incoming signals, the radar system may receive enough true returns to learn all it must about the position of the aircraft. Furthermore, such systems require the use of intermediate frequency and detector stages. The use of intermediate frequency stages limits the bandwidth of frequencies to which the system may respond and introduces further delay. Detector circuits also provide some delay and add to the complexity and cost of the system. In addition, if the radar operator changes operating frequency or "jitters" the pulse repetition frequency, the system may no longer be effective.